Proving the accuracy of flow meters is very important for suppliers of natural gas and/or any other gas which is purchased by a supplier's customers. That is, since the cost of the supplied gas is computed by the amount of gas flowing through the meter, then any meter deviation from the actual amount of gas which is supplied to the customer will result in an undercharge/overcharge for that particular customer. As may be appreciated, over time, the underchanging/overcharging could result in significant/customer, respectively.
In order to reduce such potential economic losses, it has been the industry practice for some time to periodically check the accuracy of flow meters in the field (i.e., provide stationary on-site proving apparatus at the meter in the gas pipeline). There are, however some disadvantages to these conventional meter proving systems. For example, conventional meter proving systems are usually provided for each meter in the pipeline for which proving is desired. Thus, the conventional proving system is stationary at the meter site thereby requiring a replication of such a system for each meter for which proving is desired. This, of course, results in greater costs to the gas supplier associated with proving of the supplier's meters. Hence, it would especially be desirable if a meter proving system was provided which was mobile --i.e., a system which could be dispatched and transported from one meter to be proved to another.
The present practice of proving gas flow meters is to determine as accurately as possible the actual flow rate of the gas flowing through the meter. This is usually accomplished with a high precision sonic valve array downstream of the meter. The sonic valve array will output a signal indicative of the volumetric flow rate of gas and this flow rate is then converted to a mass flow rate using well known algorithms. However, some of the variables used in these known algorithms depend upon the mass quantities of the various gas constituents flowing through the pipeline (e.g., for natural gas these constituents may be CO.sub.2, methane, ethane, propane, and others). The present practice is to predetermine the quantities of the various gas constituents in advance of meter proving, (as by submitting a sample of the gas to a laboratory for quantitative analysis) and then inputting the results of this quantitative predetermination into the proving system in the form of "constants". Obviously, the quantitative gas constituency may change between the time that the analysis is completed and the time that the meter is actually proved thereby possibly contributing to a less than accurate error factor for the meter.
It would therefore particularly be desirable in terms of accuracy if a meter proving system was provided which measures quantitatively the various gas constituents in real time (i.e., concurrently with the meter proving). Thus, if a real time quantitative gas constituent measurement was possible, then the resulting signal representative of the actual mass flow of the gas could be conditioned on the basis of the real time measured quantities thereby providing for more accurate determination of the actual gas flow rate.
According to the present invention, method and apparatus are provided which not only permit gas flow meters to be proved on-site via a mobile gas proving unit, but which also use real time quantitative measurements of the gas constituents flowing through the meter so that more accurate meter proving results.
The invention achieves these objectives by providing a self-contained meter proving system in association with a suitable vehicle. Thus, all that is needed to prove a meter in the field is to dispatch the mobile proving unit and to obtain the error factors for particular meters. These error factors can then be downloaded into the supplier's main data file, for example, so that subsequent charges for gas supplied to a customer (based on the meter's output) can be adjusted by the error factor.
A more accurate determination of the meter's error factor is achieved by providing a device which is capable of quantitatively deriving the presence of any preselected number of gas constituents. Preferably, the device employed with the present invention to accomplish this function is a gas chromatagraph which outputs a signal representative of the quantitative presence of each of a predetermined number of gas constituents in the gas flow. This outputted signal is then supplied as an input for determining mass flow data for the gas stream using well known algorithms. In such a manner, real time quantitative analysis of the gas stream flowing through the meter is obtained thereby providing a more accurate determination of the meter's error factor.
These and other advantages of the invention will become more clear to the reader after consideration is given to the following detailed description of the preferred exemplary embodiment.